1. Field of the Invention
The present invention relates to a steering control device that is provided with a steering wheel steering mechanism having a steering actuator that imparts a steering reaction force to a steering wheel, and a vehicle wheel steering mechanism having a vehicle wheel steering actuator that drives a vehicle wheel steering shaft.
The present invention can be effectively applied to various types of steering control device that are mounted in a vehicle, such as, for example, a so-called steer-by-wire system or a so-called variable gear ratio system.
2. Description of the Related Art
As an example of a conventional steer-by-wire system, art such as that disclosed in Patent Document 1 and Patent Document 2 detailed below are widely known.
FIG. 12 is a control block diagram that indicates a control method of a steering control device 900 that is a conventional steer-by-wire system.
A steering wheel steering mechanism of this steering control device 900 is fundamentally configured from a steering wheel shaft 14 disconnected from a vehicle wheel steering shaft 8; a handle (steering wheel) 1; a torque sensor (steering torque sensor) 3; a reaction force motor (steering actuator) 4; a reaction force control portion 5, as well as other members.
The reaction force control portion 5 and a position control portion 10A may be respectively configured from single processing devices (control devices), or alternatively, they may be configured from a single processing device (control device) that executes two control programs. Moreover, respective motor drive circuits, not shown, that respectively drive the reaction force motor 4 and a vehicle wheel steering motor 6, may be attached to each of the motors 4 and 6, or may be attached to the respective processing devices (control devices). Accordingly, the configuration of the above structure may, generally speaking, be selectively chosen.
The reaction force control portion 5 determines a command current in for the reaction force motor 4 based on a command current In for the vehicle wheel steering motor 6 that is determined by a predetermined feedback control (hereinafter referred to as “position control A”) executed by the position control portion 10A, and a steering torque τ output by the torque sensor 3. As a result, an optimal steering reaction force is generated.
Moreover, a vehicle wheel steering mechanism of the steering control device 900 is fundamentally configured from a steering angle sensor (steering wheel angle sensor) 2; a position sensor (steering change amount sensor) 7, and a tire 9, as well as the aforementioned position control portion 10A for executing the position control A, the vehicle wheel steering motor (vehicle wheel steering actuator) 6, and the vehicle wheel steering shaft 8.
FIG. 13 is a control block diagram that indicates a control method of the position control portion 10A of the conventional steering control device 900 described above. A command value Xn of a steering change amount of the vehicle wheel steering shaft 8 is determined so as to be substantially proportional to a steering angle θ, by a steering change amount command value calculation portion 11A for executing the position control A. A PID control portion 12 determines the command current In for the vehicle wheel steering motor 6 based on the steering change amount command value Xn and an detected value Xa of the steering change amount, using a known PID control. Thus, the direction of the tire 9 is controlled to a desired direction by execution of the position control A.
With the above control method, if, for example, the steering change, amount command value Xn exceeds an actual physical end-of-movement position (±XE) of the vehicle wheel steering shaft 8 due to execution of a large steering operation, the value of the command current In is rapidly increased in accordance with the exceeded amount. At this time, an output torque (a steering reaction force) of the reaction force motor 4 is also rapidly increased due to operation of the reaction force control portion 5. Accordingly, for example, when no physical limit is established for a rotation range (an end-of-movement or contact point) of the handle 1, the above configuration acts so as to spontaneously establish (simulate) a virtual end-of-movement for a steering range.
In other words, with the conventional control method like that described above, when, for example, no physical limit is established for the rotation range (the end-of-movement or contact point) of the handle 1, the configuration is effective in generating a virtual contact resistance force (steering reaction force) corresponding to the end-of-movement of the vehicle wheel steering shaft 8, and thus the steering angle θ is inhibited from exceeding threshold values derived from a predetermined permissible range of the steering angle θ(−θE≦θ≦θE).
(Patent Document 1) Japanese Patent Laid-Open Publication No. 2001-334947 (Paragraphs 4 and 5, FIG. 1)
(Patent Document 2) Japanese Patent Laid-Open Publication No. Hei. 05-105100 (Paragraphs 2 to 4, FIGS. 1 to 3)
However, with the aforementioned conventional method, the virtual contact resistance force (steering reaction force) in the vicinity of the end-of movement of the predetermined steering range is generated by increase of the command value Xn along with the steering angle θ, in accordance with the steering change amount Xa that is mechanically fixed by a physical end point. Accordingly, on some occasions, the command current In for the vehicle wheel steering motor 6 becomes extremely large. If this state continues for a long duration, heat build-up or damage of the vehicle wheel steering motor 6 sometimes occurs.
As a result of this problem, in the case of the conventional method, compact and lightweight manufacture of the vehicle wheel steering actuator (the vehicle wheel steering motor 6) is hindered, and thus when the above described conventional method is adopted, vehicle manufacturing cost, vehicle design flexibility, and ease of vehicle maneuver, and the like, are all disadvantageously affected.
FIG. 1 is a control block diagram showing a control method of a steering control device 100 having a heat build-up inhibition function that has been proposed in order to address heat build-up problems like that described above. A position control method of the vehicle wheel steering shaft 8 of this steering control device 100 is slightly different to that of the position control portion 10A of the aforementioned steering control device 900.
More specifically, a position control portion 10B in FIG. 1 is configured using a steering change amount command value calculation portion 11B, as shown in FIG. 2, instead of by the steering change amount command value calculation portion 11A shown in FIG. 13. Position control (position control B) of the vehicle wheel steering shaft 8 is executed as a result of operations of this steering change amount command value calculation portion 11B and the PID control portion 12. It should be noted that the PID control portion 12 and the other structural members shown in FIG. 13 are used without modification.
FIG. 2 is a graph illustrating a calculation method of the steering change amount command value calculation portion 11B of the steering control device 100. In this graph, ±XE indicates a permissible range of a steering change amount. ±XE is set in accordance with the limits of the change amount of the actual vehicle wheel steering shaft 8. In this way, for example, if the upper and lower limits of the command value Xn of the steering change amount are fixed with a guard that uses a limiter, or the like, the command current In for the vehicle wheel steering motor 6 can be inhibited from becoming excessive, and the aforementioned heat build-up problem is addressed.
However, if processing is executed with a guard of this type, the output torque (the steering reaction force) of the reaction force motor 4 is also restricted due to the operations of the PID control portion 12 and the reaction force control portion 5. Accordingly, generation (simulation) of the virtual end-of-movement for the steering range, as in the case of the steering control device 900 of FIG. 12, ceases to occur.
Moreover, generation of the contact counter force (the steering reaction force) as with the steering control device 900 also ceases. As a result, a handle degree of play that the handle 1 can easily enter becomes present as shown by the hatched portion of FIG. 2. When the steering angle θ enters this handle degree of play, linear steering feeling and responsiveness of steering control is lost.